The use of DNA sequences to estimate the timing of evolutionary events is increasingly popular.
Based on the central idea that the differences between the DNA sequences of two species are a function of the time since their evolutionary separation (Zuckerkandl and Pauling, 1965), molecular dating has been used as a method to investigate both patterns and processes of evolution (Magallón, 2004; Renner, 2005; Rutschmann, 2006; Sanderson et al., 2004; Welch and Bromham, 2005).
Geological calibrations points are assigned to phylogenetic nodes based on the assumption that a geographic barrier caused phylogenetic divergence, thus generating the risk of circular reasoning, if the chronogram derived from the calibration is used to test biogeographical scenarios (Conti et al., 2004; Magallón, 2004).
Nevertheless, geological events can provide important validation of dating estimates produced with other types of calibration (e.g., Bell and Donoghue, 2005; Conti et al., 2002; Sytsma et al., 2004).
Although the fossil record is widely regarded as the best source of nonmolecular information about the ages of selected clades (Magallón and Sanderson, 2001; Marshall, 1990b; Sanderson, 1998), several problems plague its use for calibration purposes, including (i) erroneous fossil age estimates, (ii) the idiosyncrasies of fossilization, (iii) the assignment of fossils to specific nodes in a phylogeny, and (iv) the number of fossils used for calibration.
In this paper we focus primarily on the two latter aspects of fossil calibration, although all four problems are interrelated.
These sets generated lower standard deviations associated with the ages of node X than sets characterized by lower corrected consistency.
The three calibration sets with the highest corrected consistencies produced mean age estimates for node X of 79.70, 79.14, and 78.15 My.
These timeframes are most compatible with the hypothesis that the Crypteroniaceae stem lineage dispersed from Africa to the Deccan plate as it drifted northward during the Late Cretaceous.
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